1. Field of the Invention
The present invention relates to a lens driving module which directly transmits a driving force of a piezoelectric motor to a lens barrel such that the lens barrel is prevented from being tilted in a housing.
2. Description of the Related Art
With the development of technology, the resolution of camera modules mounted on mobile terminals, camcorders and so on has changed to several million pixels. Further, various additional functions such as autofocusing and optical zoom have been added to the camera modules.
The camera module performs the autofocusing or optical zoom function by vertically transferring a lens barrel and changing a relative distance. The camera module includes a lens transfer device for vertically driving the lens barrel having lenses stacked therein.
Since such a camera module transfers the lens barrel by using an electromagnetic motor, the number of components built in the camera module increases. As a result, the size of the camera module inevitably increases.
Therefore, when the camera module is mounted in a mobile phone, for example, there are difficulties in assembling the camera module because of a limited space of the mobile phone.
FIG. 1 is a diagram showing the structure of a conventional lens driving device disclosed in U.S. Pat. No. 6,268,970. The conventional lens driving device includes frames supporting lens groups 120, 130, and 140 and cam tubes 160 and 170 supporting the frames. The respective cam tubes support the frames such that the frames can relatively move in the direction of the optical axis of an optical system, and are driven by an actuator 110.
In such a lens driving device, the relative positions of the respective lenses are determined by the shape of a cam. Therefore, a focus lens and a focus adjusting mechanism for adjusting a focus at a specific magnification are additionally required, and a driving mechanism such as a lens holding mechanism, which moves along a final reduction gear and the cam, becomes complex.
FIG. 2 is a diagram showing another conventional lens driving device disclosed in Korean Patent Laid-open Publication No. 2000-55180. The conventional lens driving device includes a fixed lens group 201 coupled to a camera body 200, the fixed lens group 201 including a plurality of lenses. The camera body 200 has a housing space formed therein, and a zoom motor 203 is housed in the housing space. The zoom motor 203 has a shaft coupled to a guide screw 205, and the guide screw 205 has a screw thread and a screw groove formed on the outer circumference thereof. Further, a clip 207 for transmitting power is coupled to the outer circumference of the guide screw 205. The clip 207 has a screw thread and a screw groove formed thereon. The screw thread and the screw groove of the clip 207 have the same shape as the screw thread and the screw groove of the guide screw 205 such that one side of the clip 207 contacted with the guide screw 205 is coupled to the screw thread and the screw groove of the guide screw 205. One side of the clip 207 is coupled to a zoom barrel 209. The zoom barrel 209 is coupled to a moving lens group 202. The zoom lens barrel 209 is coupled to a guide shaft 211 disposed in an optical-axis direction so as to move along the guide shaft in the optical-axis direction.
In the zoom lens mechanism constructed in such a manner, when the zoom motor 203 rotates, the guide screw 205 is rotated. Then, the torque of the guide screw 205 is converted into a straight-line motion by the clip 207. Therefore, the clip 207 moves straight in the optical-axis direction. As the clip 207 moves straight, the zoom barrel 209 moves along the optical-axis direction. When the zoom barrel 209 moves along the optical-axis direction, a portion of the zoom barrel 209 coming in contact with the guide shaft 211 is slid in such a manner that the zoom barrel 209 can reciprocate in the optical-axis direction.
In the zoom lens mechanism constructed in such a manner, since an electromagnetic motor is used, electromagnetic waves may occur. Therefore, the application of the zoom lens mechanism into small-sized communication devices is limited. Further, since the electromagnetic motor is used, a final reduction gear is used, so that the mechanical structure of the zoom lens mechanism becomes complex. Further, in order to adjust a focus, the zoom lens and the focus lens should be moved independently from each other.
Recently, an ultra-small optical zoom lens mechanism has been developed so as to be applied to a small optical system having a zoom function. In such an ultra-small optical zoom lens mechanism, an electromagnetic motor is not used, but an intellectual element such as a piezoelectric element is mainly used. As the electromagnetic motor is substituted with the piezoelectric element, a driving mechanism for driving a lens can be simplified, which makes it possible to achieve high efficiency.
FIGS. 3 and 4 are diagrams showing a further conventional lens driving device using such a piezoelectric element, disclosed in U.S. Pat. No. 6,215,605. In the conventional lens driving device, piezoelectric actuators 311a and 311b are fixed to base blocks 321 and 322, respectively, and their expansion and contraction are transmitted to driving rods 316 and 317. Then, lenses L2 and L4 are transferred by pre-pressure generated from sliding portions 331a and 332a and an inertia force of lens holders 331 and 332. As the lens holders are transferred or slide with the driving rods in accordance with the waveform of the pressure of the piezoelectric actuators 311a and 311b, the lenses L2 and L4 can be transferred in both directions.
When the displacement of one piezoelectric actuator between adjacent piezoelectric actuators 311a and 311b is transmitted through the base block 313, the displacement may be transmitted to another lens. Therefore, the base block 313 has a groove 313g formed therein so as to prevent the transmission of displacement between the piezoelectric actuators. As the groove is formed, the structure of the lens driving device becomes complex. Further, the displacement interference between the piezoelectric actuators cannot be perfectly removed.
Further, the length of the driving rods 316 and 317, which are moved by the piezoelectric actuators so as to transfer the lenses, is limited depending on the size of the piezoelectric actuators. The limitation of the length of the driving rods causes the limitation of lens transfer distance, thereby having an effect upon product performance.
In this case, since the driving rods are fixed, the length of a barrel having lenses built therein cannot be changed. In addition to a space for the transfer distance of the lenses, a separate space in which other components are arranged is necessary, which makes it difficult to reduce the size of the device. Further, since only one ends of the lenses are supported by the driving rods, asymmetrical displacement occurs in the lenses such that the lenses are likely to be unstably driven.
Further, since the driving rods 316 and 317 are fixed, the length of the barrel cannot be varied. Therefore, there is a limit in reducing the size of the lens driving device. Further, since an asymmetrical wave is used as a driving signal instead of a sine wave, the driving circuit becomes complex.